1. Field of the Invention
The present invention relates to a radiation imaging apparatus and a radiation imaging system which are suitably used for a diagnosis for medical use and a non-destructive test for industrial use.
2. Description of the Related Art
In a radiation imaging apparatus having a pixel array in which a plurality of pixels each including a transistor that supplies a desired potential to a conversion element for converting X-rays or light into electric charge are arranged, an electric charge which has been generated by a dark current occurring in each of the plurality of pixels is required to be reset before the radiation imaging apparatus is irradiated with a radiation. For this reason, among such radiation imaging apparatuses, there is a radiation imaging apparatus which performs a reset operation of resetting an electric charge of the pixel periodically, by sequentially setting the transistors of the plurality of pixels at a conducting state periodically to reset the conversion element, before the radiation imaging apparatus is irradiated with the radiation.
Japanese Patent Application Laid-Open No. 2010-268171 discloses a radiation imaging apparatus which detects an electric current flowing in a bias wire that supplies a bias to a conversion element therethrough while performing the reset operation, and detects an irradiation of the radiation from a radiation generating apparatus. It is also disclosed to stop a dummy read operation corresponding to the reset operation, in response to the irradiation of the radiation. When the reset operation is stopped, the operation is shifted to an accumulation operation which sets all of transistors at a non-conducting state so that the pixel accumulates the electric charge according to the irradiation of the radiation. Furthermore, after a predetermined time period (accumulation time period) has passed from the time when the accumulation operation has started, a real reading operation is started which corresponds to a read out operation of successively reading out the electric charge as a pixel signal. Here, it is disclosed to enhance reset efficiency by setting a time period during which the transistor is in the conducting state in the reset operation, so that the time period becomes longer than a conducting time period of the transistor in the read out operation.
However, when the radiation imaging apparatus is structured so as to detect a bias current while performing the reset operation and detect the start of the irradiation of the radiation, and the like, as has been described above, there exist the following problems. Specifically, a part of a signal electric charge which has been generated by the irradiation of the radiation results in being lost from the conversion element in which the reset operation has been performed in a period between the start of the irradiation of the radiation and the detection of the start of the irradiation of the radiation. Because of this, as the conducting time period of the transistor in the reset operation is longer, the influence becomes larger. For this reason, it is desirable to shorten the conducting time period of the transistor in the reset operation, in order to obtain an image having a small artifact. Specifically, it is desirable to shorten a time period during which the transistors in all of the pixels are sequentially reset at the conducting state to be reset in the reset operation (hereinafter referred to as time cycle of reset operation).
However, on the other hand, when the time cycle of the reset operation is shortened in order to obtain the image having the small artifact, there arises a problem that detecting capability at the time when the radiation imaging apparatus detects the irradiation of the radiation results in being lowered, as has been described above.